Britain's government must give official guidance on when it is legal to shoot and kill maritime pirates, MPs said on Thursday ahead of a conference in London on the future of Somalia.

Parliament's foreign affairs committee hailed the government's decision in October to allow British merchant ships in dangerous waters to carry armed guards to protect them from pirates.

The report said piracy was a "major problem for the UK and the international community" and that more than $300 million (230 million euros) had been paid in ransoms to Somali pirates over the past four years.

But the committee said the government's guidance on the use of lethal force remained "very limited" and that ships' captains needed to know what they could do if they were threatened by pirates.

"The question anyone would ask is that if a private armed guard on board a UK flagged vessel sees an armed skiff approaching at high speed, can the guard open fire?" committee chairman Richard Ottaway said.

"The government must provide clearer direction on what is permissible and what is not."

Ottaway said it was "unacceptable that 2.6 million square miles (6.7 million square kilometres) of the Indian Ocean has become a no-go area for small vessels, and a dangerous one for commercial shipping".

Britain is to host an international conference on Somalia in London next month.

Prime Minister David Cameron announced on October 30 that he was relaxing a legal ban so that British-flagged ships can apply for a licence to carry weapons on board in the most dangerous areas.

The government said the areas likely to be covered would be the coast of Somalia, in the Gulf of Aden, in the Arabian Sea and the wider Indian Ocean.